ABSTRACT Background and Objectives: Nowadays, infections with antibiotic-resistant bacteria are among the most important causes of mortality worldwide. This has attracted the attention of researchers to seek suitable alternatives for antibiotics. The venom of many toxic species such as arthropods has antibacterial properties. In this study, we investigated antibacterial effects of crude venom of Latrodectus dahli on Escherichia coli, Staphylococcus aureus, and Bacillus subtilis. Methods: Lyophilized crude venom of L. dahli was dissolved in 50 mM Tris-HCl buffer. Protein concentration was determined by the Bradford assay. Then, the bacteria were exposed to different concentrations (31.25-250 ng/mL) of the crude venom. Inhibitory activity of the venom against the bacteria was determined by MTT assay and determining minimum inhibitory concentration (MIC). Results: Results of the MTT assay showed that the crude venom significantly inhibited the growth of E. coli (31.25 and 62.5 ng/mL), S. aureus (at 250 ng/mL) and B. subtilis (at 125 and 250 ng/mL). In the MIC experiment, the crude venom significantly inhibited the growth of E. coli (at concentrations of 31.25 and 62.5ng/mL), S. aureus (at concentrations of 31.25-250 ng/mL) and B. subtilis (at concentrations of 31.25-250ng/mL). Conclusion: The crude venom of L. dahli and its components showed relatively strong antibacterial effects. Keywords: Spider venoms, Black Widow Spider, Antibacterial agent, Drug-resistance.

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